Optical films are used to modify the transmission, reflection, and absorption properties of an optical device. Common functions provided by optical films include the redirection of light, the provision of angularly selective transmission, reflection, and absorption, and the selective transmission of one polarization state of light. A common method of producing optical films is to form a structured surface on a film that either refracts or scatters light, or both. Methods for producing these types of films include microreplication and the provision of coatings that form a structured or optically active surface. Another method for making an optical film is to generate a composite film made from materials that have different optical properties. Examples of these types of films include coextruded polymeric layers that form interference filters and extruded blends of two different polymers to provide a randomly positioned discontinuous phase within a continuous phase.
Composite films are capable of providing complex optical functions that are highly desirable in many applications. A practical limitation of composite films, however, is that it is difficult to precisely position elements in more than one dimension. This lack of precision reduces the overall optical performance and cosmetic quality of optical films.